The image of UC Davis police calmly coating seated protesters with pepper spray in November was shocking to the millions who saw it on videos gone viral - but it wasn't a criminal act, the Yolo County district attorney has concluded.

At issue was whether officers broke the law on Nov. 18 by using the chemical irritant on 21 students during a protest over rising tuition.

In a 12-page report issued this week, the district attorney's office describes UC police trying to remove a group of arrested students while surrounded by protesters chanting, "Set them free!" A smaller group then linked arms and sat down across the walkway, blocking the officers' exit route.

In the videos, Lt. John Pike of the campus police is seen slowly and deliberately spraying the students, pausing to shake the can before continuing. At least one other officer, Alexander Lee, is also seen spraying.

By linking arms, the seated protesters engaged in "active" rather than "passive" resistance, according to the district attorney's report, which relied not only on the officers' accounts but on University of California-commissioned studies of the incident, on experts in the use of force, and on UC policy.

Officers may use pepper spray when they encounter active resistance, the report says.

But whether the officers used the chemical in an unlawful manner - 11 students were treated for the effects of pepper spray and two were taken to the hospital - could not be proved "beyond a reasonable doubt," says the report.

"Lt. Pike's pepper spraying of the seated protesters has been seen by and has outraged millions of viewers throughout the world," the report says, but adds that the decision about whether to prosecute considered a broader context, including the officers' belief, discredited in hindsight, that they were surrounded by a hostile mob.

There were between 200 and 250 protesters, said Mike Cabral, assistant chief deputy of the district attorney's office.

Pike and Lee, who had been on paid leave, left their jobs in July. Pike's attorneys did not return calls for comment. But Pike told the Sacramento Bee that he was relieved that he would not be charged. After the incident, Pike had been deluged with harassing phone calls, e-mails and visits to his home.

UC officials declined to comment, as did several students who were pepper-sprayed.

The 21 are in the process of settling their civil lawsuit against UC, which the regents approved Sept. 13. The American Civil Liberties Union, representing the students, expects to file the settlement in federal court in Sacramento next week. The terms will then become public, said spokeswoman Rebecca Farmer.

This incident and the forceful use of batons on students by UC Berkeley police in November prompted UC officials to examine their response to protests across the 10-campus system. UC spent nearly $1 million on two studies, outside attorneys and insurance, said Steve Montiel, a UC spokesman.